Looks good - at least, the implementation (omitting the fact that personally I don't know absolutely anything about Japanese )
Regarding the functions, it's recommended to set pd->nUnload explicitely at the end of the function body, e.g.

in case of a modeless dialog. Otherwise you may encounter unexpected behaviour and even a crash.
Though I did not find such assignments in the standard Format plugin, and it looks strange to me Better ask Instructor about this.

2. You are using Unicode literal(L-strings) explicitly, so you must use Unicode variants of API calls explicitly too(i.e you must SendMessageW, MessageBoxW, wsprintfW instead of SendMessage, MessageBox, wsprintf etc)
3. You have converted AkelDll.h into unicode(utf8). Unfortunately not all C-compilers support Unicode-encoded sources. MSVC <7.0/7.1 doesn't, so I could not compile it. About source file ConvertDll.c - you saved it in 932 codepage(Shift-JSI).. unfortunately MSVC 9.0 cannot recognize it correctly, but after conversion into utf8 it works.
4. More about Unicode: I'll advice you not to use Unicode symbols in sources directly, because of encoding problems(see 3). It is good idea to use them indirectly by means of \x#### escape sequences - they are portable(\u is not valid sequence), whereas direct symbols in some environments could be displayed incorrectly(for instance: I see them as boxes, because of my system hasn't CJK monospace fonts(Arial Unicde MS has CJK subset but it is not monospace so source is unreadable).
5. You used MSVC 10 (2010) to develop and build plugin. You have linked against shared version of libc library (msvcr100.dll). But you didn't provided us with SxS metadata: embedded manifest or manifest file with SxS dependencies. So plugin does not loads at all. You should create proper manifest or link against static version of libc(or do not use libs at all)
6. I think it is better to define FullKata2FullHira, HiraToFullkata and ___htmlEncode as function not as macros. Variant as function is easier to support, IMHO.